# Scalar form factors of light mesons

Ananthanarayan, B and Caprini, I and Colangelo, G and Gasser, J and Leutwyler, H (2004) Scalar form factors of light mesons. In: Physics Letters B, 602 (3-4). pp. 218-225.

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The scalar radius of the pion plays an important role in $\chi$PT, because it is related to one of the basic effective coupling constants, viz. the one which controls the quark mass dependence of $F_\pi$ at one loop. In a recent paper, Yndurain derives a {\it robust lower bound} for this radius, which disagrees with earlier determinations. We show that such a bound does not exist: the derivation'' relies on an incorrect claim. Moreover, we discuss the physics of the form factors associated with the operators $\bar{u} u$,$\bar{d} d$ and $\bar{s} s$ and show that their structure in the vicinity of the $K\bar{K}$ threshold is quite different. Finally, we draw attention to the fact that the new data on the slope of the scalar $K_{\ell 3}$ form factor confirm a recent, remarkably sharp theoretical prediction.